The influence of mechanical adhesion of copper coatings on carbon surfaces on the interfacial thermal contact resistance

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1 Thin Solid Films 433 (2003) The influence of mechanical adhesion of copper coatings on carbon surfaces on the interfacial thermal contact resistance a,b, a b b c c E. Neubauer *, G. Korb, C. Eisenmenger-Sittner, H. Bangert, S. Chotikaprakhan, D. Dietzel, c c A.M. Mansanares, B.K. Bein a ARC Seibersdorf Research GmbH, Dept. Materials and Production Engineering, Seibersdorf A2444, Austria b Vienna University of Technology, Institute of Solid State Physics Thin Film Group, Vienna, Austria cruhr Universitat Bochum, Exp.Phys.III, Solid State Spectroscopy Group, D Bochum, Germany Abstract The weak mechanical and thermal interface in Copper based Metal Matrix Composites (MMCs) reinforced by carbon fibers is the background of this research study. In order to investigate the mechanical adhesion strength and the thermal contact resistance between copper and carbon, a simplified model system based on coated flat carbon substrates has been used. Cu coatings of approximately 1 mm thickness on intermediate Ti bond layers have been deposited on carbon substrates by a sputter deposition process. The resulting different adhesion strengths between coating and substrate have been measured by means of a pull-off method, and non-destructive photothermal method, giving information on the depth-resolved thermal properties of the samples has been used to determine the Thermal Contact Resistance (TCR). By combining the results of mechanical adhesion tests and of photothermal IR radiometry, the correlation of the mechanical adhesion of Cu coatings on Carbon surfaces with the interfacial thermal contact resistance has been analysed Elsevier Science B.V. All rights reserved. Keywords: Thermal Contact Resistance (TCR); Adhesion; Interface; Copper; Carbon; Metal matrix composites 1. Introduction Metal matrix composites are a special type of materials, combining the properties of a metallic matrix and the physical properties of the reinforcement material. The reinforcement can rely either on particles or on fibers. In the case of copper carbon composites one of the advantages would be a high thermal conductivity combined with a low Coefficient of Thermal Expansion (CTE). Due to a weak interfacial contact between copper and the carbon fibers, which is related to the absence of any reactivity or dissolution between the two constituents, weak mechanical bonding is usually observed. Additionally, the thermal gap at the interface contributes to reduce the thermal transport inside the composite and thus limits the application of such metal carbon fiber composites for thermal management problems. *Corresponding author. Tel.: q ; fax.: q address: erich.neubauer@arcs.ac.at (E. Neubauer). The main focus in this research is on the investigation of mechanical and thermal interfacial properties in this material system. By combining the results of adhesion tests and photothermal measurements, a correlation between the adhesion strength and the thermal transport properties is expected w1x. In order to confirm this assumption, investigations have first been done on simplified model systems consisting of Cu-coated flat carbon substrates instead of Cu-coated carbon fibers. Various samples have been prepared and tested in order to get basic information on the Cu C interface and to test the sensitivity of the two selected measurement techniques. In this article, first results are presented, which have been obtained by frequency-dependent thermal wave radiometry giving depth-resolved information on the thermal transport inside the Cu C model system in general and information on the Thermal Contact Resistance (TCR) between the copper coatings and carbon substrates in special. These results are compared with the measurements of mechanical adhesion strength /03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 E. Neubauer et al. / Thin Solid Films 433 (2003) Fig. 1. Schematical set-up of a pull-off testing equipment for the determination of the mechanical adhesion strength (1. Stamp; 2. Viton-ring; 3. Sample; 4. Frame; 5. 3M Adhesion Foil). 2. Experimental Since there exist several types of carbon fibers w2x, a special type of carbon substrate has been chosen for this basic research study: glassy carbon, which has a very smooth surface and shows isotropic properties due to its amorphous structure w3x. Thus, effects of a meso- or macroscopic surface roughness, which might contribute to improved mechanical or thermal contact properties, are eliminated or reduced Sample preparation and mechanical adhesion tests To overcome generally low adhesion between Cu and C and in order to get significant differences in the adhesion strength, samples with tailored mechanical interfaces have been prepared. This was realized by several steps of pre-treatment of the substrates and by intermediate bond layers leading to improved adhesion of the coatings. Both the intermediate bond layers consisting of Ti and the copper layers have been deposited by sputtering w4x and different procedures to clean the substrates have been applied. For the preparation of the samples a sputter deposition chamber described in Ref. w5x has been used. The adhesion strength of the copper coatings have been measured by using a pull-off test realized by a system similar to a tensile test equipment, with a stamp glued on the coated surface of the samples by means of a special 3M adhesion foil. The force to be applied at the stamp during the pull-off test is then measured and used to determine the adhesion Fig. 2. Set-up for photothermal measurements. strength. The schematic of the pull-off test system is shown in Fig. 1. For mechanical and thermal characterization, four samples with different adhesion strengths have been prepared: one sample with the copper coating directly deposited on a received carbon substrate (S63); one sample with the copper coating deposited on a cleaned carbon substrate (S62); and two samples with an intermediate Ti bond layers of 5 nm between copper coating and substrate (S64a, S64b). For all samples the thickness of the copper coating was approximately 1 mm. Table 1 gives an overview of the measured adhesion strength of the copper coatings, 2 which varies between approximately 5 Nycm and about Nycm Measurement of the thermal transport properties For the measurement of the thermal transport properties and the determination of the Thermal Contact Resistance (TCR) a photothermal system (Fig. 2) has been applied, which consists of four main components: i. an Argon ion laser is used for periodically modulated heating of the sample; Table 1 Scaling of the ratio of the calibrated normalized amplitudes at small and large penetration depths with the mechanical adhesion strength determined by pull-off method Sample Sample description Symbol in Fig. 3 S n(f 0) S (f `) S63 Cu coating on uncleaned substrate S62 Cu coating on cleaned substrate n S64b Cu coatingqti layer on cleaned substrate = S64a Cu coatingqti layer on cleaned substrate e n 2 Adhesion strength wnycm x

3 162 E. Neubauer et al. / Thin Solid Films 433 (2003) ii. an IR system consisting of IR optics and a HgCdTe detector is used to measure the IR radiation emitted by the sample surface; iii. a two-phase Lock-in amplifier is used for electronic filtering of the detector signal at the frequency f of modulated heating and gives separate information on the amplitude and phase shift of the thermal wave response; and iv. the sample is kept in High-Temperature Cell enabling temperature-dependent measurements at average sample temperatures between 20 8C and C. The measurement system enables excitation and detection of thermal waves in the heating modulation frequency range of 0.01 Hz-f-100 khz. This allows depth-resolved measurements of thermal properties in the range from approximately 1 micrometer to some millimeters. The detection limits and further details of the measuring system are described in Ref. w6x. For these TCR measurements modulated heating is applied at the surface of the Cu coatings, leading to periodical thermal waves that propagate across the coating, the interface, and the substrate. Partial reflection of the thermal wave occurs at the interface and, from the amplitude and phase shift of the thermal waves measured by contactless IR radiometry at the surface of the heated coating, a quantitative description of the interfacial thermal resistance can be derived. For the interpretation of the measurements it is necessary to have information on the substrate material. To this finality, additional measurements have been performed on various reference samples (cleaned and uncleaned glassy carbon). In Fig. 3 the signal amplitude of thermal waves are shown, which have been measured as a function of frequency at an average sample temperature of approximately 25 8C for two reference samples and four Cucoated carbon samples with different bonding conditions. The two reference measurements (h,s) of glassy carbon (Sigradur) and of the cleaned carbon substrate decrease with 1yyf in the frequency range of 2 Hz- f-20 khz, which is a characteristic for one-dimensional thermal wave propagation in semi-infinite solids (Fig. 3). Due to the finite response time of the detector, the decrease of the reference amplitudes (h,s) are slightly stronger at higher frequencies, 30 khzff. In comparison with the reference signals (h,s), the amplitude of the samples S62 (n) and S63 ( ) show a stronger variation with frequency (Fig. 3), which is a characteristic for a two-layer system with a stronger contrast in the effective thermal properties of the two layers, respectively for a contrast between coating and substrate which is increased by a thermal contact resistance. In sample S63 ( ) the thermal contact resistance Fig. 3. Thermal wave amplitudes as function of frequency measured at an average sample temperature of approximately 25 8C for reference samples and Cu-coated carbon samples with different bonding conditions: h glassy carbon (Sigradur), s cleaned carbon substrate, sample S63 (Cu coating on uncleaned carbon), n sample S62 (Cu coating on cleaned carbon), = sample S64b (Cu coating Ti bond layer carbon), e sample S64a (Cu coating Ti bond layer carbon). seems to be larger than in sample S62 (n). The amplitude of the samples S64a (e) and S64b (=) also deviate from the 1yyf decay of the homogeneous ref- erence samples (h,s), in comparison with the samples S62 and S63, however, the deviations are less pronounced, indicating intermediate bond layers between Cu coatings and substrates. 3. Interpretation of thermal measurements For the quantitative interpretation of the measurements, it is preferable to calibrate the signals measured for the coated samples by using the signals measured for a homogeneous body of smooth surface as reference. By such a calibration procedure, the frequency characteristics of the measurement device can be eliminated and the thermal properties of the coated body can be compared with the thermal properties of the reference material. In the following, the signals measured for the cleaned carbon substrate are used as reference for calibration. In the calibrated and additionally normalized form of Fig. 4, the signals S s(f ) dt s(f ) n S r(f ) dt r(f ) S (f )A A (1) give information on the relative depth distribution of the thermal wave amplitudes, independent of the optical surface properties (absorptance and emissivity). In Eq. (1), the quantity S corresponds to the photothermal s

4 E. Neubauer et al. / Thin Solid Films 433 (2003) In Fig. 5 the calibrated phase signals w sw yw, n s r with w, the phase shift measured for the coated samples s and wr that of the cleaned carbon substrate as reference, are plotted as function of the square root of the heating modulation frequency. In this representation, the small penetration depths are on the right-hand side and the large penetration depths are on the left-hand side of Fig. 5. The measured data are compared with the theoretical approximations for a two-layer model with an additional thermal contact resistance Rth characterized by the boundary conditions of continuity of heat flux and discontinuity of the temperature at the transition between Cu coating and carbon substrate, T 1(d c)yt2 Rths (3) yk T y zz 1 1 zsd c In Eq. (3), T 1(d c) is the temperature of the coating and yk1 T1y z± zsdc, the heat flux at the transition between coating and the substrate. As can be seen in Fig. 5, reasonable agreement between measured data and theoretical approximation can be obtained in the range of intermediate frequencies, Fig. 4. Calibrated signal amplitudes S n (Cu-coated samplesycleaned carbon reference) after additional normalization on the value 1 at large penetration depths plotted vs. (fyhz). sample S63 (Cu coating on uncleaned carbon substrate), n sample S62 (Cu coating on cleaned carbon substrate), = sample S64b (Cu coating Ti bond layer cleaned carbon), sample S64a (Cu coating Ti bond layer cleaned carbon). signal measured for the coated sample while, Sr repre- sents the signal measured for the reference substrate, and the quantity dt s(f ) is the thermal wave amplitude in the coated sample and dt r(f ) that in the homogeneous reference material. As can be seen from Fig. 4, the temperature equilibration and heat transport are most inhibited in sample S63 ( ), consisting of a Cu coating on the uncleaned carbon substrate, while the heat transition from the coating to the substrate is less inhibited in sample S64a (e), consisting of Cu coating, Ti bond layer, and cleaned carbon substrate. In order to quantify the effect of the thermal contact resistances Rth on the temperature equilibration and the heat transport in the different samples, the thermal wave amplitudes at small penetration depths y2 xa(ayf) A(fyHz) f10 (2) are compared with the value 1 of the calibrated and normalized amplitudes at large penetration depths, (f y y1 Hz) f2=10. The results obtained from Fig. 3 are presented in Table 1. Fig. 5. Calibrated phase signals w n (Cu-coated samples cleaned carbon reference) plotted vs. (fyhz), sample S63 (Cu coating on uncleaned carbon substrate), n sample S62 (Cu coating on cleaned carbon substrate), = sample S64b (Cu coating Ti bond layer cleaned carbon), e sample S64a (Cu coating Ti bond layer cleaned carbon), in comparison with the theoretical approximations for a two-layer model with additional thermal contact resistance 2 y6 y6 Rthy(m KyW)s4=10 (broken line), 6=10 (continuous), y6 9=10 (broken-pointed line).

5 164 E. Neubauer et al. / Thin Solid Films 433 (2003) e.g. for sample S62 (n) in the range between 8-(f y Hz) -100 with a thermal contact resistance of Rths y =10 m KyW. The deviation between the measured data and the theoretical approximations at higher frequencies, corresponding to smaller penetration depths, are probably due to the deposition conditions of the coating, resulting in a thermally inhomogeneous Cu layer. The deviations at low frequencies, corresponding to large penetration depths, are due to 3-dimensional heat transport, which is not considered in the theoretical approximation of Fig. 5. Three-dimensional heat transport becomes relevant when the thermal diffusion length is equal to or larger than the size of the heating spot, mths(ay f) Gd spot. By comparing the phase signals of the different coatings in Fig. 5, one can see clear differences in the maxima of the normalized phases: (i) With improved adhesion and decreasing thermal contact resistance, the phase maxima decreases from wnmaxf298 for sample S63 (Cu coating on uncleaned carbon substrate), to wnmaxf258 for S62 (Cu coating on cleaned carbon), and to wnmaxf158 for sample S64a (Cu coating Ti bond layer cleaned carbon). Simultaneously, the phase maxima shift with improved adhesion to higher values of the heating modulation frequency, e.g. from f maxf1.2 khz for S63 to fmaxf2 khz for S64a, corresponding to effectively a smaller thermal penetration depths m th s (ayf) of the thermal contact resistance. If we compare the phase maxima and the corresponding modulation frequencies measured for the two samples S64a and S64b, we can see that for these two samples, prepared with Ti bond layers of approximately 5 nm under identical process parameters, the scattering of these results are negligible (Fig. 6). As shown in theoretical simulations of relatively thin coatings of good thermal conductivity on carbon substrates, it is difficult to distinguish between the effect of the thermal contact resistance and the thermophysical properties of the coating, which vary with the thickness and can considerably deviate from literature data due to the deposition conditions w7x. For a more reliable determination of the thermal contact resistance in the presence of such thin sputter-deposited Cu coatings, the effective thermal properties of the coating have to be measured separately, preferentially at even higher modulation frequencies than used in this work and by using a method where excitation and detection spots are separated and where thus the lateral heat transport can be measured w8x. 4. Conclusions The measurement of thermal waves based on IR detection is an effective tool to get information about the thermal transport properties between PVD copper Fig. 6. Relative calibrated normalized thermal wave amplitudes Sn and maxima of the normalized phase w n, as function of the mechanical adhesion strength. coatings and carbon substrates. Based on these first results, a correlation between the thermal contact resistance and the mechanical adhesion strength in a coppercarbon system has been found. The results show that there is a positive effect on the mechanical adhesion strengths that simultaneously has beneficial effects for the thermal transfer between the copper coating and the substrate, when thin bond layers of Ti are introduced between coating and the substrate. Based on the results obtained until now, we expect a thermal contact resistance, which exponentially decreases with the improved adhesion strength. Further measurements and testing of samples prepared with alternative substrate preparation procedures are still necessary for a more complete characterization of the Cu C interface. Quantitative information about the thermal contact resistance between coating and substrate can be useful for simulation and modeling of complex structures such as metal matrix composites where, the interface between the matrix and fiber bundles plays an important role. Once a reliable correlation between thermal and mechanical properties have been established for a certain type of metal matrix carbon system, photothermal characterization can be used as an indirect but powerful tool for non-destructive measurements of the mechanical interface properties. Acknowledgments This work is a result of the project network Interface Optimisation in Metal Matrix Composites which is supported by the Austrian Fonds zur Forderung wissenschaftlicher Forschung (grants P14534-PHY and P15116).

6 E. Neubauer et al. / Thin Solid Films 433 (2003) References w1x D. Sakami, A. Lahmar, Y. Scudeller, F. Danes, J.P. Bardon, Thermal contact resistance and adhesion studies on thin copper films on alumina substrates, J. Adh. Sci. Technol. 15 (12) (2001) w2x J.-B. Donnet, Carbon Fibers, 2nd, Marcel Dekker Inc, New York, w3x Leaflet HTW Hochtemperaturwerkstoffe GmbH (data-sheet w4x E. Neubauer, C. Eisenmenger-Sittner, H. Bangert, G. Korb, Proceedings of 13th International Colloquium on Plasma Processes, CIP 2001 (June 10 14, 2001) Antibes-Juan-les-Prins, (Ed. SFV, France). w5x C. Eisenmenger-Sittner, H. Hutter, Microchem. Acta 133 (2000) w6x J. Bolte, J.H. Gu, B.K. Bein, Background fluctuation limit of IR detection of thermal waves at high temperatures, High Temp.-High Pressures 29 (5) (1997) w7x S Chotikaprakhan, D Dietzel, J Pelzl, B.K. Bein, Simulation of photothermal experiments on Cu Carbon interface systems (2003), (to be publ. in J. Appl. Phys.). w8x D. Dietzel, S. Chotikaprakhan, E. Neubauer, B.K. Bein, J. Pelzl, Thermoreflectance analysis of sputter-deposited thin copper films on carbon, (to be publ. in Proceedings of 14th International Colloquium on Plasma Processes), CIP 2003 (June/July, 2003) Antibes-Juan-les-Prins, (Ed. SFV, France).

AFM and AUGER investigations ofas-deposited and heat treated copper coatings on glassy carbon surfaces with titanium intermediate layers

Vacuum 71 (2003) 293 298 AFM and AUGER investigations ofas-deposited and heat treated copper coatings on glassy carbon surfaces with titanium intermediate layers E. Neubauer a,b, *, C. Eisenmenger-Sittner